1. The Field of the Invention
The present invention relates to methods and devices for achieving and maintaining controlled atmospheres, and the devices in which such atmospheres are maintained. In particular, the present invention is related to producing and maintaining a controlled reducing atmosphere in a field emitter device.
2. The Background Art
Field emitter devices (FEDs) are under study for a variety of uses, including the production of flat panel displays (FPDs). These displays are presently under development to provide, for example, flat television screens.
A FED is generally produced by sealing two parallel, closely spaced, planar glass members along their perimeters. Typically, the sealing is performed by melting a glass paste having a low melting point along one or both of the perimeters of the two glass members and bringing the members together to sealably join them along their perimeters, a method known commonly as "frit sealing". The resulting structure consists of two parallel glass surfaces separated by an interior space a few hundreds of microns (.mu.m) in width. The interior space of the FED typically is kept under vacuum.
On the inner surface of one glass member is positioned a plurality of pointed microcathodes (microtips) made of a metallic material, e.g., molybdenum (Mo), which emit electrons. A plurality of grid electrodes are placed proximate to the cathodes on the same surface so as to generate a very high electric field. On the opposing glass surface are deposited phosphors. The electric field created by the arrangement of grid electrodes and microtips ejects electrons from the points of the microtips and accelerates the electrons toward the phosphors, exciting the phosphors into luminescent states. The luminescence intensity of the excited phosphors, and, therefore, the pixel brightness, is directly proportional to the current emitted by the associated microtips.
Until now it was considered necessary to keep the pressure of the interior space below about 1.times.10.sup.-5 millibar (mbar) to achieve good luminescence intensity. To this end several workers have proposed the use of getter materials, such as BaAl.sub.4, (see, e.g., European Patent Application Serial No. EP-A-443865), in addition to metals such as tantalum (Ta), titanium (Ti), niobium (Nb) or zirconium (Zr) as described in European Patent Application Serial No. EP-A-572170. Powdered Ti, Zr, thallium (Th) and their hydrides have also been combined with Zr-based alloys and employed in the shape of porous layers as described in Italian Patent Application Serial No. M194-A-000359. Each of the above-cited patent applications is incorporated herein by reference.
Recent studies, however, suggest that not all the gases present in the interior space have a detrimental effect on the performance of the FED. In particular, hydrogen may be present in the device at pressures higher than about 1.times.10.sup.-5 mbar. Spindt et al. in IEEE Transactions on Electron Devices 38(10): 2355-2363, 1991, and Mousa in Vacuum 45(2-3):235-239, 1994, have shown that hydrogen does not substantially affect the electronic emission, even for long periods, if the hydrogen is present at a pressures less than 1.5.times.10.sup.-2 mbar. Both of the references cited above are incorporated herein by reference in their entirety and for all purposes. Furthermore, introducing hydrogen into an "aged" FED, i.e., a FED whose electronic emissivity has decreased over time, restores the emissivity to its initial value. Spindt has also shown that oxidizing gases, in particular air, have the expected negative effect on the current emission from the microtips. Mousa further points out that the presence of hydrogen at pressures higher than 2.times.10.sup.-1 mbar in the interior space also has a negative effect on the electronic emissivity, probably due to the erosion of the microtips resulting from their bombardment by hydrogen ions at these relatively high pressures. Thus, these studies together suggest that a gaseous environment inside the FED should be one that is relatively free of oxidizing gases and contains a small partial pressure of a reducing gas, in particular hydrogen.
Although the beneficial effects of hydrogen are generally known, there is at present no industrially useful method for controlling the amounts of hydrogen and oxidizing gases within the interior space of a FED. The academic studies performed to date have followed laboratory procedures in which hydrogen is introduced into the FED through a suitable conduit ("tail") formed in the structure of the FED itself and attached to an external hydrogen source. Unfortunately, such laboratory procedures are not readily applicable to the industrial production of FEDs. In particular, the introduction of low partial pressures of hydrogen into the space through an external hydrogen source is difficult to control reproducibly. In addition, local heating caused by the "tip off" process, in which the tail is closed by heating, can cause significant hydrogen leakage from the interior space. Finally, laboratory methods do not provide a practical method for maintaining the reducing atmosphere in the FED over its lifecycle.
Thus, it would be advantageous to provide a method for creating and maintaining a reducing atmosphere inside the interior space of a FED. It would also be advantageous to provide a FED capable of maintaining a reducing atmosphere throughout its lifecycle. In particular, it would be desirous to provide an atmosphere in a FED substantially free of oxidizing gases and including a reducing gas, such as hydrogen.